A stupid question abour GPWS

Mahesh

New Member
Hey guys,
I was thinking earlier today. Does the Ground Proximity Warning System go off when a plane is about to land? Sort of like how the stall warning goes off on GA aircraft during the flare.
If not, how does the system know that the proximity to the ground is legit and not unintended? Is it based on some average calculation of sink rate or rise in terrain?

Thanks,
Mahesh
 

Fearless

Dash Dominatrix
Mahesh,

Not a stupid question at all! The aircraft I fly has a very sophisticated EGPWS (Enhanced GPWS) which monitors altitude, speed, sink rate, gear and flap position, proximity to obstacles and terrain, deviation from the glideslope, and other factors. We've had several crews "unintentionally" activate the system by flying along at the minimum altitudes on an instrument approach with the gear and/or flaps up. (I put "unintentional" in quotes because the crews could have probably avoided the GPWS alert by slowing down and having the aircraft configured.)

The easiest way to "set off" the system on short final is to get below the glideslope, OR to get an excessive sink rate (which could be caused by windshear or pilot error). The "below glideslope" warning requires that you have the localizer frequency tuned up. If you fly a stabilized approach and stay at or above the glideslope (if used), then you should not receive any GPWS alerts.

FFFI
 

Mahesh

New Member
Thanks Fearless!

I was thinking that there would be many parameters that are used by the system to figure out if there is danger, but I would have never thought of all those items!

Mahesh
 

A300Capt

Freight Dawg
The old system assumed once the gear and flaps were out that the pilots were landing regardless of whether or not there was actually a runway ahead of them. About the only warning you would get is "sink rate" if certain parameters were exceeded.

The new EGPWS (enhanced) system uses and incorporates the worldwide terrain data base in the FMC. It knows where the aircraft is in relation to the airport and surrounding terrain(assuming no map shift or other abnormality). You have terrain floor protection while on the approach even with the gear and flaps down. The system is pretty cool and the "painting of terrain" feature on the moving map is used when flying into or out of a mountainous airport and gives plenty of warning in order to maneuver away from any danger. Man made obstacles, such as radio towers, are now being incorporated into the FMC's data base.
 

Fearless

Dash Dominatrix
Louie,

I fly the Dash 8-400 ("D") model, a 70-seat version of the Dash 8. It's a turboprop, but cruises along at around 360 kts TAS. Both the captain and I have our own multifunction displays and primary flight displays. In flight, the multifunction displays normally depict navigation data and terrain (a "moving map"), while the primary flight displays show attitude, altitude, airspeed, vertical speed, etc. We also have a large engine display in the center of the panel which shows all the engine parameters (torque, rpm, NH, NL, oil temp and pressure, fuel temp, etc.).

The multifunction displays can be toggled between the navigation data and aircraft system displays. If you want to find out "what the electrical system is doing", you can jump over the the electrical page and inspect the generator and battery loads, etc. We also have the capability to display weather radar (instead of terrain) on top of the navigation page. It's pretty common to see one pilot flying along with the terrain display up on his side, and the other guy with the radar selected.

The terrain mapping feature described by A300Capt is almost identical to what we have in the Dash. The terrain around the airplane is displayed as a colored map, with red and yellow representing higher terrain and shades of black and green used for lower terrain. Oceans are shown in blue. The detail is truely amazing - it looks a little like a sectional chart. We can also superimpose symbols for airports, VORs, and other aircraft.

As A300Capt said, this system is extremely useful when operating into mountainous terrain, especially during night or instrument conditions. It's extremely cool watching the terrain on the display change from green and black to yellow and red as you descend into a mountainous airport (say, Sun Valley).

Hope this wasn't too technical.

FFFI
 

chperplt

New Member
<<Sort of like how the stall warning goes off on GA aircraft during the flare.>>

As you get more experience you'll realize that the stall horn shouldn't be going off in your ears on landing. Passengers won't enjoy that very much.
 

EatSleepFly

Well-Known Member
[ QUOTE ]
As you get more experience you'll realize that the stall horn shouldn't be going off in your ears on landing.

[/ QUOTE ]

Heh, try telling THAT to a few of the old-school crusty instructors!


I do agree though...unless its a short-field or very nasty/windy, I'll sacrafice runway (within reason, of course) to make a smooth landing with passengers on sightseeing flights and such.
 

Louie1975

Well-Known Member
Thanks Fearless! I actually would love to one day fly an advanced turboprop. I am a private pilot. I got interested in them when I flew on ATRs on CSA Czech Airlines. Too bad not too many US Airlines are ordering turboprops anymore.
 

Mahesh

New Member
[ QUOTE ]
<<Sort of like how the stall warning goes off on GA aircraft during the flare.>>

As you get more experience you'll realize that the stall horn shouldn't be going off in your ears on landing. Passengers won't enjoy that very much.

[/ QUOTE ]

Ha ha ha this is true! I haven't carried but a couple of people when I have flown.

What I was taught was that the stall warning should go off right before the mains touch down. In my case, I am pretty inconsistent. Sometimes, I can hear the blaring horn. In other cases, it never goes off


I went flying with a United pilot once and he landed, he had the nose up in the air and the stall warning was blaring for several seconds. He put it down right on the center line though.

Mahesh
 
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